Method for passivating a metallic surface with a basic composition

ABSTRACT

A method for passivating a metallic surface, by treating the surface with a basic aqueous composition comprising one polymer comprising acidic groups, a cationic crosslinker and a volatile basic compound, allows a durable passivation of fragile metal surfaces.

The present invention relates to a method for passivating a metallicsurface using an aqueous, basic composition which comprises a polymercomprising acidic groups, a cationic crosslinker, and a volatile basiccompound.

The present invention further provides an aqueous, basic composition forpassivating a metallic surface, the composition comprising a polymercomprising acidic groups, a cationic agent, and a volatile basiccompound. The invention also relates to the use of the composition forpassivating a metal surface.

The present invention likewise relates to a coating on a metallicsurface obtainable by the inventively described method.

Metallic materials, more particularly iron and steel, are typicallygalvanized in order to protect them from corrosive environmentalinfluences. The corrosion control afforded by the zinc derives from thefact that it is baser than the metallic material itself, and thereforeis initially corroded itself. Since the zinc layer itself is alsosubject to corrosion (white rust), the corrosive attack on a zinc coatof this kind is frequently retarded by the application of what is calleda passivation coat. The passivation coat shall retard the corrosiveattack on the metal surface and may at the same time serve to provide animproved adhesion of any coats of coating material that are to beapplied. Instead of the term “passivating coat”, the term “conversioncoat” is frequently used synonymously.

The application of passivating coats takes place, for example, forgalvanized metal parts (e.g., electrogalvanized or hot dip galvanizedsupports) which are subsequently coated. Application also takes placefor parts which are employed without coating. Similarly, metallicsurfaces of aluminum or of aluminum alloys are frequently provided witha passivating coat, especially when they are to be coated subsequently.

The raw material used for the production of sheetlike metallicworkpieces, such as, for example, automobile parts, bodywork parts,equipment linings, façade claddings, ceiling claddings or windowprofiles, are at present typically long metal strips which, by means ofsuitable techniques, are shaped into the desired parts and/or joined.

The corrosion control treatment of metallic materials of these kinds isaccomplished typically in multistage operations. The surface of treatedmetals often has a number of different layers. A corrosion controltreatment may be performed at various points in the productionoperation. This corrosion control may be temporary or permanent.Temporary control, for example, is applied only for the storage ortransportation of the metallic workpiece, such as the metal strip, forexample, and is removed again before ultimate processing.

Of particular technical and economic importance are strips having agalvanized surface, more particularly strips of electrogalvanized or hotdip galvanized iron or alloys of iron such as steel, for example. Alsoof importance are metal strips comprising aluminum or alloys ofaluminum.

In general, one or more additional coating-material coats are applied tothe passivated surface. Examples of the purposes of thesecoating-material coats are to protect the passivation coat and the metalfrom corrosive gases and/or liquids, and/or from mechanical damage (suchas stone chipping, for example). Such a coat may also serve estheticpurposes. Coating-material coats are typically much thicker thanpassivation coats. Typical thicknesses for a coat of coating materialrange from 4 μm to 400 μm.

In the prior art, passivation coats on surfaces of zinc or of aluminumhave been obtained typically by treating the workpiece to be protectedwith aqueous acidic solutions of chromates (e.g., CrO₃) or with acidicaqueous solutions of Cr(III) salts (see EP-A 0 907 762).

In the presence of atmospheric oxygen, the surface of zinc or alloys ofzinc or of aluminum or alloys of aluminum in general first develops athin oxide layer which retards corrosive attack on the underlying metal.Typically in the passivating methods based on chromium compounds, thisexisting oxide film, along with some of the metal to be protected, isdissolved, and is incorporated at least partly into a film on the metalsurface.

This film resembles the naturally existing oxide film and in generalcomprises specifically introduced phosphate, heavy metals and/orfluorides. The resulting passivation or conversion coats shalleffectively protect the underlying metal from corrosive attack.

More recently, passivation methods, for metal-coated steel panels andpiece goods (e.g., hot dip galvanized steel), for example, have alsobeen developed that are based on polymers as organic film-formers. Inthese methods, usually acidic aqueous solutions of various film-formingpolymers, containing carboxyl, phosphoric acid and/or phosphonic acidgroups, for example, are used to form the passivation coat. Followingthe application of the acidic formulation, there is typically partialdissolution of the metal surface (e.g. zinc) and polyvalent metal ions(e.g., Zn²⁺) are released. Typically there is an increase in pH in thevicinity of the metal surface. This leads normally to crosslinking andfilm-forming of the acidic polymers with the polyvalent metal ions.

Since, with the passivation method described, using acidic polymers, theuse of heavy metals such as chromium is avoided, these polymer-basedpassivation methods are gaining steadily in importance. Known in theprior art are various—mostly acidic—passivating preparations, which ingeneral comprise a water-soluble, film-forming polymer containing acidgroups. Methods for passivation using these preparations have alsoalready been described.

DE-A 195 16 765 relates to a method for generating conversion coats onsurfaces of zinc or of aluminum by treatment with an acidic solutionwhich comprises an organic film-former and also aluminum ions in theform of a water-soluble complex with chelate-forming carboxylic acidsand phosphoric acids. Organic film-formers cited includecarboxyl-containing polymers, more particularly homopolymers and/orcopolymers of acrylic and/or methacrylic acid.

WO 2004/074372 describes the passivation of metallic surfaces usingcopolymers comprising acrylic acid and vinylphosphonic acid and/ormaleic acid, the passivating formulation possibly comprising furthercomponents.

WO 2008/012248 describes acidic preparations for passivating metallicsurfaces, comprising copolymers synthesized from monomethacrylic estershaving hydrophobic groups (e.g., hydroxyethyl acetate), monomers withphosphonic acid groups (e.g., vinylphosphonic acid), and monomers withcarboxyl groups (e.g., acrylic acid).

WO 2006/134116 describes a method for passivating metallic surfaces bytreatment thereof with an aqueous composition comprisingacid-group-containing polymers and polyvalent cations such as zinc,calcium, magnesium or aluminum ions.

WO 2006/134117 describes a method for passivating metallic surfaces bytreatment thereof with an aqueous composition comprising acidicpolymers, with addition of waxes (e.g., polyethylene waxes).

Aqueous passivating solutions of this kind, for the purpose of improvingthe corrosion control, are often applied to the galvanized steel stripdirectly after the galvanizing line (e.g. after hot dip galvanizing).Application is generally accomplished by means of roll technology, with,for example, simple squeeze rolls or more technologically sophisticatedroll coaters being employed.

In the case of the squeeze roll, which is technologically simple andinexpensive to realize, the passivating solution is applied to thegalvanized steel strip (by spraying, for example) and then squeezed offusing a roll. Using the squeeze-roll technology it is frequentlypossible only to obtain substandard, i.e., less uniform, passivationcoats. If the applied coats are subject to particular requirements, themore sophisticated and more costly roll-coater technology is employed,where the passivating solution is first applied to one or more rolls andthen transferred to the steel strip. With this method it is usuallypossible to obtain more homogeneous passivation coats with a relativelyuniform thickness.

In general the time available for film-forming, i.e., crosslinking, bythe acidic polymer is comparatively short. In the case of the continuouspassivation of a steel strip, the time remaining from the application ofthe formulation to the steel strip until the drying of the coated stripin the drier, depending on line speed, is normally just a fewseconds—typically, for example, 2 to 10 seconds.

WO 2009/047209 describes a continuous method for the coating of steelstrips, where the passivating composition is first applied to a coatingroller, taken off by a doctor blade, and then transferred to the steelstrip.

In zincked metal surfaces there is frequently a certain fraction ofaluminum present (e.g., in a hot dip galvanized coating, around 0.2% Alto 99.8% Zn). In passivating formulations described in the prior art,therefore, fluorides are frequently added as adjuvants. In an acidicmedium, fluoride can act as a complexing agent for the aluminum oxide(Al₂O₃) present on the surface. Fluorides are undesirable on account oftheir environmental properties (toxicity) and the costly andinconvenient wastewater treatment that results.

Typically the passivating methods described in the prior art and usingan acidic, aqueous formulation based on polymers containing acid groupsentail the crosslinking of the acid-group-containing polymer from insideto outside (for instance, by zinc ions extracted at the metal surface),i.e., from the metal surface toward the air/film layer interface. As aresult of this, there is often an incompletely crosslinked uppermostpolymer layer formed, which, accordingly, remains water-soluble. It canbe detached; the corrosion resistance decreases and the surface exhibitsan unattractive appearance. These inadequately crosslinked passivatinglayers give rise, for example, to unfavorable values in the stack test.In said test, the coated metal surfaces are stacked atop one another.The stacks are then exposed to water over a period of several days.

It is an object of the present invention to provide compositions andmethods for the treatment of metal surfaces that are suitable for theformation of a passivation coat on metal surfaces, the intention beingto achieve a stable passivating film coat that as far as possible isconsistently crosslinked. The passivation coats ought, moreover, tooffer enhanced corrosion control for the metal surface.

A further object of the present invention is to provide a fluoride-freepassivating composition.

The passivation coat obtained is to be suitable not only for workpieceswhich are subsequently coated but also for those which are employedwithout coating. In this context, in the event of subsequent coating ofthe workpieces, it must be borne in mind that sufficient adhesion of thesubsequent coating-material coat to the passivation coat is ensured.This means that the adhesion of coating material to the passivation coatshould improve (or at least not deteriorate). The quality ofcoating-material adhesion can be determined using cross-cut test (seealso DIN ISO Standard 2409) on a planar metal surface and/or on a metalsurface with defined denting. For this purpose, a defined cross-cutpattern is insized into the coating down to the coated substrate. Adefined adhesive tape is then placed over the cross-cut and peeled off.The delamination of the coated cross-cut elements is then evaluatedvisually and typically reported using characteristic cross-cut values,on a scale from 0 to 5.

Also of importance are the optical qualities of polymer-containingpassivation coats, with clear and transparent coats being desired. Theoptical qualities may be adversely affected by the phenomenon known as“chalking”. When affected by this phenomenon, the coats are no longercompletely clear and transparent, but instead more or lessnontransparent, white spots. “Chalking” can easily be confused with theformation of white rust, and may make quality control more difficult.Therefore, the passivation coats produced with the method of theinvention or by means of the composition of the invention ought to haveclear, transparent optical properties and a low propensity towardchalking.

It has surprisingly been found that on a metallic surface, moreparticularly on metallic surfaces comprising zinc and/or aluminum, apassivating, corrosion-inhibiting polymer layer is formed if polymerscomprising acidic groups are applied to the metal surface in the form oftheir aqueous (clear), alkaline solution (more particularly at a pH of9.5 to 11), the solution further comprising at least one volatile base,more particularly ammonia, and at least one cationic crosslinker (e.g.,polyvalent metal ions, more particularly Zn²⁺, and/or a polyamine).

With the method of the invention it is possible to obtain highly stable,thoroughly crosslinked passivation coats which in particular exhibitgood corrosion resistance in the stack test.

The passivating composition of the invention, moreover, is preferablyfree from heavy metal compounds, more particularly chromium compounds,and from fluorides. Under alkaline conditions no additional complexingagent is needed in order to dissolve the aluminum oxide (Al₂O₃) layer.

A further advantage is the paint adhesion on the metallic surfacescoated with the method of the invention, said adhesion being improvedrelative to unpassivated metallic surfaces and/or to passivationcoatings of the prior art.

Coat formation in the method of the invention may take place withcrosslinking of the acidic polymers with the cationic crosslinkers as aresult of a reduction in pH, following the volatilization of thevolatile base, for example. In a strongly alkaline medium, the cationiccrosslinkers are present in deprotonated form or as salt of oxo acidsand/or hydroxo acids (e.g., as zincate).

A reduction in pH may lead to the release of cationic charge (e.g., Zn²⁺or polyammonium ions) and to crosslinking with the acidic groups,present in deprotonated form, of the film-forming polymer. Thecrosslinking therefore may take place both from outside to inside, i.e.,starting from the air/water film phase boundary, and from inside tooutside, i.e., starting from the metal surface/water film phaseboundary. The latter may take place as a result of the alkalinedissolution of the metal, e.g., of Al and Zn, at the metallic surface,with evolution of hydrogen and with a drop in the pH.

The crosslinking of hydrophobic, acrylate-containing polymer dispersionswith polyvalent cations such as zinc ions and the addition of ammoniaare known in floor care/floor sealing, among others. U.S. Pat. No.3,308,078 describes a composition for coating various surfaces,comprising a polymer emulsion with an organic film former, ammonia, anda metal-releasing complex.

The present invention pertains to a method for passivating a metallicsurface, in particular a metallic surface essentially consisting of oneor more metals selected from the group consisting of zinc (Zn), aluminum(Al), and magnesium (Mg), wherein a metallic surface is contacted withan aqueous composition comprising (or consisting of) the followingcomponents:

-   -   a) at least one water-soluble polymer (X) comprising acidic        groups, the polymer (X) having at least 0.6 mol of acid        groups/100 g of polymer; more particularly, the acidic groups        are selected from carboxyl groups, sulfonic acid groups,        phosphoric groups and/or and phosphonic acid groups;    -   b) at least one volatile basic compound (B), preferably selected        from ammonia, C₁₋₆ alkyl amines and C₁₋₆ alkanolamines;    -   c) at least one cationic crosslinker selected from polyvalent        metal ions (M) and cationic polymers (P), more particularly        cationic polyamines;    -   d) at least one solvent (L), comprising at least 80% by weight        of water;    -   e) optionally one or more surfactants (T);    -   f) optionally further components (K);    -   the pH of the aqueous composition being in the range from 9 to        12, preferably in the range from 9.5 to 11.

The present invention pertains more particularly to a method forpassivating a metallic surface, as described above, wherein acomposition is employed comprising (or consisting of):

-   -   a) 10% to 40% by weight of at least one water-soluble        polymer (X) comprising acidic groups, the polymer (X) having at        least 0.6 mol of acid groups/100 g of polymer;    -   b) 1% to 20% by weight of at least one volatile basic compound        (B), preferably selected from ammonia, C₁₋₆ alkylamines, and        C₁₋₆ alkanolamines;    -   c) 0.01% to 25% by weight of at least one cationic crosslinker        selected from polyvalent metal ions (M) (preferably Mg²⁺, Ca²⁺,        Zn²⁺, Mn²⁺, Fe²⁺, Cr³⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺, Ce³⁺, Ce⁴⁺, more        preferably Zn²⁺) and cationic polymers (P), more particularly        cationic polyamines (more particularly selected from        tetraethylenepentamine, polyethyleneimines, polyethyleneimine        derivatives, polyvinylamines, and polyimidazoles,    -   d) 20% to 89% by weight of at least one solvent (L), comprising        at least 80% by weight of water;    -   e) optionally 0% to 3% by weight of at least one surfactant (T),    -   f) optionally 0% to 30% by weight of at least one further        component (K),    -   the pH of the aqueous composition being in the range from 9 to        12, preferably in the range from 9.5 to 11.

The invention also relates to the formulations and compositionsthemselves and to their preparation.

The Polymer (X)

For passivation by means of the method of the invention, an aqueouscomposition is used which comprises at least one water-soluble polymer(X) comprising acidic groups. The polymers (X) employed may behomopolymers or copolymers. It is also possible for mixtures of two ormore different polymers to be employed.

The formulation employed in accordance with the invention comprisespreferably 10% to 40%, more preferably 15% to 35%, very preferably 15%to 30%, and frequently 18% to 25% by weight of the polymer or polymers(X), based on the amount of all of the components in the formulation(including the solvents).

The term “water-soluble” in the sense of this invention means that thepolymer or polymers (X) employed are homogeneously water-soluble, in theamounts stated for the compositions. The polymers (X) employed oughtpreferably to be of infinite (unrestricted) miscibility with water. Moreparticularly, the polymers employed ought to have a solubility of atleast 50 g/l, preferably 100 g/l, and more preferably at least 200 g/l,in water at room temperature and a pH of 7. It should be borne in mind,however, that the solubility of the polymers (X) described, comprisingacidic groups, in water is also dependent on the pH. A polymer which atone particular pH has an insufficient solubility for the intendedpurpose may, at another pH, have a solubility which is still sufficient.

In particular, the polymers (X) employed contain at least 0.6 mol ofacid groups/100 g of polymer. The polymers preferably contain at least0.9 mol of acid groups/100 g, more preferably at least 1 mol of acidgroups/100 g, and frequently in fact at least 1.2 mol of acid groups/100g.

The acidic groups of the polymers (X) are generally selected fromcarboxyl groups, sulfonic acid groups, phosphoric acid groups and/orphosphonic acid groups. The acidic groups are preferably selected fromcarboxyl groups, phosphoric acid groups, and phosphonic acid groups.With particular preference the polymer (X) employed is a copolymer (X1)synthesized from at least two different monomers containing acid groups,more particularly from monomers comprising carboxyl groups and frommonomers comprising phosphonic acid groups.

For performing the invention it is particularly preferred to usehomopolymers or copolymers which comprise acrylic acid units and/ormethacrylic acid units.

More particularly the polymer (X) comprises one or more water-solublecopolymers (X1) synthesized from acrylic and/or methacrylic acid monomerunits (M1) and from monoethylenically unsaturated monomers with acidicgroups (M2), these monomers being different from (M1). Optionally,furthermore, there may be OH-containing acrylic esters or methacrylicesters (M3) present. Optionally there may be further monomers (M4)present as structural units.

In one preferred embodiment of the invention, the water-soluble polymer(X) is a copolymer (X1) which is synthesized from (or which comprises)the following monomers:

-   M1: 30% to 90% by weight of methacrylic acid and/or acrylic acid;-   M2: 10% to 70% by weight of at least one further monoethylenically    unsaturated monomer, different from (M1), which has one or more    acidic groups (more particularly monoethylenically unsaturated    dicarboxylic acids having 4 to 7 carbon atoms, monoethylenically    unsaturated phosphoric acids, monoethylenically unsaturated    phosphonic acids, preferably monoethylenically unsaturated    phosphonic acids); optionally M3: 0% to 40% by weight of at least    one OH-containing methacrylic ester and/or acrylic ester;-   optionally M4: 0% to 30% by weight of at least one further    ethylenically unsaturated monomer, different from (M1), (M2), and    (M3).

These % by weight figures are based on the sum total (100% by weight) ofall of the monomers in the copolymer (X1).

Furthermore, the preferred polymers (X) described in WO 2009/047209 thatcomprise acidic groups may be used for the purposes of the presentinvention. For the polymers (X) employed in the method of the inventionit is possible to use the embodiments described in WO 2009/047209 inrelation to the monomers (M1), (M2), (M3), and (M4).

The amount of acrylic acid and/or methacrylic acid (M1) in the copolymer(X1) is 30% to 90%, preferably 40% to 80%, and more preferably 50% to70%, by weight, this figure being based on the sum total of all of themonomers in the polymer.

The amount of the monomers (M2) in the copolymer (X1) is 10% to 70%,preferably 20% to 60%, and more preferably 30% to 50%, by weight, basedin each case on the sum total of all of the monomers in the polymer.

The monomer (M2) comprises at least one monoethylenically unsaturatedmonomer which is different from (M1) but is copolymerizable with (M1)and which contains one or more acidic groups, the acidic groups beingselected from carboxyl groups, phosphoric acid groups, phosphonic groupsor sulfonic acid groups. It is possible to use two or more differentmonomers (M2).

Concerning the preferred embodiments of monomer (M2), reference is madeto document WO 2009/047209.

Examples of such monomers (M2) comprise crotonic acid, vinylacetic acid,C₁-C₄ monoesters of monoethylenically unsaturated dicarboxylic acids,styrenesulfonic acid, vinylsulfonic acid, allylsulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), vinylphosphonicacid, monovinyl phosphate, maleic acid, fumaric acid or itaconic acid.It is preferred to use vinylphosphonic acid as monomer (M2).

The copolymer (X1) may further comprise, optionally, at least oneOH-containing acrylic ester and/or methacrylic ester as monomer unit(M3). The monomers in question are preferably monohydroxy acrylic estersand/or monohydroxy methacrylic esters. It is preferred to usehydroxyethyl acrylate as monomer (M3).

The amount of the monomers (M3) in the copolymer (X1) is 0% to 40%,preferably 1% to 30% by weight.

With regard to the further preferred embodiments of polymer (M3),reference is made to the document WO 2009/047209.

Besides the monomers (M1), (M2), and, optionally, (M3), it is possibleoptionally for 0% to 30% by weight of at least one further ethylenicallyunsaturated monomer (M4), different from (M1), (M2), and (M3), to beemployed. Furthermore, preferably, no other monomers are employed.

The monomers (M4) may serve to fine-tune the properties of the copolymer(X1). It is also possible for two or more different monomers (M4) to beused. They are selected by the skilled person in accordance with thedesired properties of the copolymer, with the proviso that they must becopolymerizable with the monomers (M1), (M2), and (M3). They arepreferably monoethylenically unsaturated monomers. In special cases,however, small amounts of monomers having two or more polymerizablegroups may also be employed. By this means the copolymer can becrosslinked to a small extent. Examples of suitable monomers (M4)comprise, in particular, aliphatic alkyl esters of (meth)acrylic acid,such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl(meth)acrylate or 2-ethylhexyl (meth)acrylate. Additionally suitable arevinyl ethers or allyl ethers such as, for example, methyl vinyl ether,ethyl vinyl ether, propyl vinyl ether, 2-ethylhexyl vinyl ether, vinylcyclohexyl ether, vinyl-4-hydroxybutyl ether, decyl vinyl ether,2-(diethylamino)ethyl vinyl ether, 2-(di-n-butylamino)ethyl vinyl etheror methyldiglycol vinyl ether, and the corresponding allyl compounds.Likewise possible for use are vinyl esters such as, for example, vinylacetate or vinyl propionate. It is also possible to use basiccomonomers, such as acrylamide and alkyl-substituted acrylamides, forexample.

Examples of crosslinking monomers comprise molecules having two or moreethylenically unsaturated groups, examples being di(meth)acrylates suchas ethylene glycol di(meth)acrylate or butane-1,4-diol di(meth)acrylateor poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate orelse di(meth)acrylates of oligoalkylene or polyalkylene glycols, such asdi-, tri- or tetraethylene glycol di(meth)acrylate. Further examplescomprise vinyl (meth)acrylate or butane diol divinyl ether.

The term “(meth)acryl” as used herein refers to either an acrylic groupor a methacrylic group.

The amount of all of the monomers (M4) employed, together, is 0% to 30%by weight, based on the total amount of the monomers employed. Theamount is preferably 0% to 20%, more preferably 0% to 10%, by weight. Ifmonomers (M4) with crosslinking activity are present, their amount oughtin general not to exceed 5%, preferably 2%, by weight (based on thetotal amount of all of the monomers used for the method). The amount maybe, for example, 10 ppm to 1% by weight.

Concerning the preferred embodiments of monomer (M4), reference is madeto the document WO 2009/047209.

One preferred embodiment relates to the above-described method whereinthe water-soluble polymer (X) is a copolymer (X1) which is synthesized(or which comprises) the following monomers:

-   -   M1: 20% to 60% by weight of acrylic acid;    -   M2: 20% to 60% by weight of vinylphosphonic acid;    -   M3: 1% to 40% by weight of hydroxyethyl acrylate.

The polymers (X) and copolymers (X1) may be prepared by methods known tothe skilled person. The copolymers are prepared preferably by radicalpolymerization of the stated components (M1), (M2), and, optionally,(M3) and/or (M4) in aqueous solution. Details relating to the conduct ofa radical polymerization are known to the skilled person. Preparationprocesses for the copolymers (X1) are described in, for example, WO2006/021308 or WO 2006/134116.

The synthesized copolymers (X1) can be isolated from the aqueoussolution by means of typical methods known to the skilled person, as forexample by evaporating the solution, spray drying, freeze drying orprecipitating. Preferably, the copolymers (X1), after thepolymerization, are not isolated from the aqueous solution; instead, theresultant solutions of the copolymers are used as they are (optionallyafter addition of further adjuvants) for the method of the invention. Inorder to facilitate such direct further use, the amount of the aqueoussolvent used for the polymerization ought to be made from the start suchthat the concentration of the polymer in the solvent is suitable for theapplication. It is also possible first to prepare a concentrate, whichis diluted only on site with water or, optionally, other solvents to thedesired concentration.

The molecular weight, more particularly based on the weight-averagemolecular weight M_(w), of the polymers (X) and copolymers (X1) used forthe method of the invention is specified by the skilled person inaccordance with the desired application. Use may be made, for example,of polymers having molecular weight M_(w) of 3000 to 1 000 000 g/mol.Especially established are polymers with 5000 g/mol to 500 000 g/mol,preferably 10 000 g/mol to 250 000 g/mol, more preferably 15 000 to 100000 g/mol, and very preferably 20 000 to 75 000 g/mol.

The Volatile Basic Compound (B)

A volatile basic compound for the purposes of the present invention is avolatile organic or inorganic compound which in aqueous solution gives abasic reaction. The vapor pressure can be employed as a measure of thevolatility.

For the purposes of the present invention, a volatile basic compound(e.g., in aqueous solution) has a vapor pressure of more than 0.01 kPa(20° C.). More preferably, a volatile basic compound for the purposes ofthe present invention has a vapor pressure of more than 0.05 kPa (20°C.), preferably of more than 0.1 kPa (20° C.), preferably of more than0.5 kPa (20° C.).

In the present invention it is possible to use the following volatilebasic compounds: the primary, secondary, and tertiary amines having aboiling point of 100° C. or less, such as ammonia (in the form ofaqueous ammonia), monomethylamine, dimethylamine, trimethylamine,monopropylamine, dipropylamine, tripropylamine, and monopentylamine, andthe like.

More particularly the volatile basic compound (B) may be at least onecompound selected from the group consisting of ammonia, C₁₋₄ alkylamines(e.g., monomethylamine, dimethylamine, trimethylamine, ethylamine) andC₁₋₄ alkanolamines (e.g., mono-, di-, and triethanolamine,1-aminopropane-2-ol). It is preferred to use ammonia as volatile basiccompound (B).

The aqueous composition used in accordance with the invention comprises1% to 20%, preferably 1% to 15%, more preferably 1% to 10%, and verypreferably 2% to 5% by weight of the volatile basic compound (B), basedon the amount of all of the components of the aqueous composition(including the solvents).

The aqueous composition used in the method of the invention has a pH inthe range from 9 to 12, preferably in the range from 9.5 to 11. The pHof the aqueous composition is dependant on the nature and concentrationof the polymers (X) used and also on the volatile basic compound (B).The pH may also be adjusted or influenced by further basic or acidiccomponents in the composition.

The Cationic Crosslinker

The aqueous composition used in the method of the invention comprises atleast one cationic compound which is able to function as a crosslinkerfor the polymer (X) comprising acidic groups. As cationic crosslinker itis possible more particularly to use at least one polyvalent cationand/or at least one cationic polymer (P), more particularly at least onecationic polyamine.

In one preferred embodiment the invention pertains to a method asdescribed above wherein the aqueous composition comprises as cationiccrosslinker at least one polyvalent metal ion (M) selected from thegroup consisting of Mg²⁺, Ca²⁺, Zn²⁺, Mn²⁺, Fe²⁺, Cr³⁺, Al³⁺, Ti⁴⁺,Zr⁴⁺, Ce³⁺, and Ce⁴⁺, preferably Zn²⁺, Mg²⁺, Ca²⁺, Mn²⁺, and Al³⁺.Preferably these ions are Zn²⁺ and/or Mg²⁺, and very preferably Zn²⁺.

Besides these, the preparation preferably comprises no further metalions. The ions may be present in the form of hydrated metal ions, butmay also be present in the form of dissolved compounds, examples beingcomplex compounds. The ions may in particular have complex bonds to theacidic groups of the polymer. If present, the amount of polyvalent metalions selected from the group consisting of Zn²⁺, Mg²⁺, Ca²⁺, Mn²⁺, andAl³⁺ is 0.01% to 25%, preferably 0.5% to 10%, and more preferably 1% to5%, by weight, based in each case on the overall composition.

As cationic polymer (P) use is made in particular of a polyamine. Apolyamine or a polyamine compound for the purposes of the presentinvention is a saturated, open-chain or cyclic organic compoundcomprising at least two (preferably at least five) amino groups, theamino groups being selectable from primary, secondary, and tertiaryamino groups. Depending on the prevailing conditions (pH), the aminogroups may be in protonated form and may carry cationic charge.

In the method of the invention, it is preferred for at least onepolyamine to be selected from the group consisting of polyalkylimines(polyiminoalkylenes, e.g., polyethyleneimine PEI), alkoxylatedpolyethyleneimines, polyvinylimidazoles (polyvinylimidazoles),polyvinylamines, and quaternized and/or alkoxylated derivatives of theabovementioned polyamines.

In a further preferred embodiment, the invention pertains to a method asdescribed above wherein the aqueous composition comprises as cationiccrosslinker at least one cationic polymer (P) selected frompolyethyleneimines, polyethyleneimine derivatives, polyvinylamines, andpolyvinylimidazoles.

In particular it is also possible for alkoxylated, preferablyethoxylated and/or propoxylated, polyamine compounds to be used in thecontext of the present invention. Use may be made more particularly ofalkoxylated polyamine compounds comprising 1 to 1000, preferably 1 to100, preferably 1 to 50, preferably 1 to 10 alkoxy units for thepurposes of the present invention.

In particular it is also possible to use quaternized polyamine compoundsfor the purposes of the present invention, said compounds having atleast one quaternary ammonium group which may be obtained in particularby substitution on the amino group by one or more radicals selected fromC₁-C₆ alkyl and benzyl, starting from the unquaternized polyaminecompound.

As cationic crosslinker in the method of the invention it is preferredto use at least one polyamine (P) selected from the group consisting ofpolyalkylimines, (polyiminoalkylenes, e.g., polyethyleneimines PEI),alkoxylated polyalkylimines (e.g., alkoxylated polyethyleneimines),quaternized polyalkylimines (e.g., quaternized polyethyleneimines),polyvinylimidazoles (polyimidazoles), quaternized polyvinylimidazoles,alkoxylated polyvinylimidazoles, polyvinylamines, quaternizedpolyvinylamines, and alkoxylated polyvinylamines.

In the aqueous composition used in the method of the invention, thecationic polymer (P) may be present in an amount of 0.01% to 25%, moreparticularly of 0.5% to 10%, preferably 2% to 10%, more preferably 2% to6%, by weight.

As cationic polymer (P) it is possible in particular to use apolyalkylimine, such as polyethyleneimine (PEI) or polypropyleneimine.In one preferred embodiment the aqueous composition comprises at leastone polyethyleneimine.

Polyethyleneimines may be described in particular by the structural unitwith the following general formula (P1)

where

-   -   R^(a) is selected from hydrogen, C₁₋₆ alkyl, benzyl or a radical        based on ethyleneimine such as —(CH₂CH₂NH)_(n).H with n′ being 1        to 200    -   x is 4 to 20 000, preferably 4 to 1000, preferably 4 to 300,        preferably 4 to 100, more preferably 10 to 100, preferably 10 to        30.

The molecular weight of the polyethyleneimines used may be situated, forexample, in the range from 100 to 800 000 g/mol, preferably in the rangefrom 100 to 50 000 g/mol, preferably in the range from 500 to 10 000g/mol, preferably in the range from 500 to 5000 g/mol.

The figures may be based in each case on the number-average molecularweight, on the weight-average molecular weight, or on theviscosity-average molecular weight; in particular, the figures are basedon the number-average molecular weight.

The polyethyleneimines in question may in particular be branchedpolyethyleneimines. Also possible is the use of alkoxylatedpolyethyleneimines (e.g., ethoxylated or propoxylatedpolyethyleneimines), preferably having 2 to 20 alkoxy units.

As polyamine component (P) it is possible in particular to usepolyvinylamines. Polyvinylamines may be described in particular by meansof the structural unit of the general formula (P2)

where

-   -   y in particular is an integer from 4 to 10 000, preferably from        10 to 5000, more preferably from 10 to 1000;    -   R^(b) and R^(c) independently of one another are selected from        hydrogen, C₁₋₆ alkyl, and benzyl.

For the purposes of the present invention it is branched polyvinylaminesthat are used in particular.

The molecular weight of the polyvinylamines used is situated inparticular in the range from 100 to 500 000 g/mol, preferably from 500to 250 000 g/mol, preferably in the range from 1000 to 250 000 g/mol.The figures may be based on the number-average molecular weight, on theweight-average molecular weight, or on the viscosity-average molecularweight; in particular, the figures are based on the number-averagemolecular weight.

In a further embodiment of the invention, it is possible as polyaminecomponent (P) to use polyvinylimidazoles. Polyvinylimidazoles may bedescribed in particular by a structural unit of the general formula(P3):

where

-   -   z in particular is an integer from 4 to 10 000, preferably from        10 to 5000, more preferably from 10 to 1000.

The molecular weight of the polyvinylimidazoles used is situated inparticular in the range from 200 to 1 000 000 g/mol, preferably in therange from 1000 to 500 000 g/mol, preferably in the range from 1000 to200 000 g/mol. The figures may be based in each case on thenumber-average molecular weight, on the weight-average molecular weightor on the viscosity-average molecular weight; in particular, the figuresare based on the number-average molecular weight.

In one preferred embodiment, the aqueous composition described abovecomprises a quaternized polyvinylimidazole. Quaternization may beaccomplished in particular by addition of a C₁₋₆ alkyl radical and/or abenzyl radical to at least one of the nitrogen atoms of thepolyvinylimidazole; in particular, the quaternization may take placewith a common methylating reagent (e.g. methyl halides).

It is further advantageous to use a combination of polyamine (P) andpolyvalent metal ion (M) as cationic crosslinker in the method describedabove.

In a further preferred embodiment, the aqueous composition used in themethod described above comprises as cationic crosslinker at least onepolyamine (P) as described above in an amount of 0.01% to 25% by weight,(more particularly from 0.5% to 10%, preferably 2% to 10%, morepreferably 2% to 6%, by weight) and at least one polyvalent metal ionselected from the group consisting of Mg²⁺, Ca²⁺, Zn²⁺, Mn²⁺, Fe²⁺,Cr³⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺, Ce³⁺, and Ce⁴⁺, (preferably Zn²⁺, Mg²⁺, Ca²⁺,Mn²⁺, and Al³⁺) in an amount of 0.01% to 25% by weight, (preferably 0.5%to 10% by weight and more preferably 1% to 5% by weight).

The Solvent (L)

Water is preferably used exclusively as solvent (L) in the compositions.Besides water, the solvent may comprise small amounts of water-miscibleorganic solvents. Small amounts of organic solvents can be used inparticular, selected from monoalcohols such as methanol, ethanol orpropanol, higher alcohols such as ethylene glycol or polyether polyols,ether alcohols such as butyl glycol or methoxypropanol, andN-methylpyrrolidone. As a general rule, the amount of water is at least80% by weight (based on the total amount of solvent), preferably atleast 90% by weight (based on the total amount of the solvent), andfrequently at least 95% by weight (based on the total amount ofsolvent).

The solvent is present in the above-described aqueous compositionpreferably in an amount of 20% to 89% by weight, preferably of 20% to80% by weight, preferably of 20% to 70% by weight, preferably of 20% to60% by weight, preferably of 20% to 50% by weight.

The Surfactants (T)

A surfactant (T) may be present optionally in the aqueous compositionused in the method of the invention, in an amount of 0% to 1% by weight,preferably 0% to 0.5% by weight. It is possible more particularly to useanionic, nonionic and/or cationic surfactants.

In one embodiment of the invention, the aqueous composition describedabove comprises 0.1% to 2% by weight, preferably 0.1% to 0.8% by weight,of at least one surfactant selected from the anionic, cationic, andnonionic surfactants known to the skilled person.

The aqueous composition used in the method described above mayoptionally comprise at least one surfactant (T), preferably selectedfrom the group consisting of:

-   i) alkoxylated alcohols, more particularly alkoxylated (preferably    ethoxylated) C₆₋₂₀ alcohols comprising 2 to 14 alkylene oxide units    (more particularly ethoxy and/or propoxy units), e.g., hexanol    ethoxylates-   ii) alkyl sulfates, more particularly C₈₋₂₀ alkyl sulfates (e.g.,    2-ethylhexyl sulfate, dodecyl sulfate)-   iii) alkylsulfonates, more particularly C₈₋₂₀ alkylsulfonates (e.g.,    dodecylsulfonate)-   iv) alkylaryl sulfates, more particularly C₈₋₂₀ alkylaryl sulfates    (e.g., cumene sulfate)-   v) alkylarylsulfonates, more particularly C₈₋₂₀ alkylarylsulfonates,    e.g., alkylbenzenesulfonates (e.g., dodecylbenzenesulfonates)-   vi) cationic surfactants, more particularly C₁₀₋₁₆    alkyl-trimethylammonium salts.

The Further Components (K)

As further components, the above-described aqueous composition maycomprise at least one of the following components:

-   a. Phosphate ions, more particularly in an amount of 0% to 10%,    preferably of 0.1% to 5%, by weight;-   b. solubilizers (anionic, nonionic, cationic), more particularly in    an amount of 0% to 1% by weight;-   c. organic crosslinkers; e.g., polyamines; including those in their    protonated form; more particularly in an amount of 0% to 10% by    weight;-   d. defoamers (e.g., silanes, modified silanes), more particularly in    an amount of 0.001% to 0.1% by weight;-   e. deaerating agents (e.g., long-chain alcohols), more particularly    in an amount of 0.001% to 0.1% by weight;-   f. activators (e.g., nitrate, nitrobenzenesulfonate), more    particularly in an amount of 0% to 2% by weight, preferably of 0% to    0.5% by weight;-   g. hydrogen scavengers (e.g., hydroxylammonium salts, hydrogen    peroxide (H₂O₂), nitrate), more particularly in an amount of 0% to    2% by weight, preferably of 0% to 0.5% by weight;-   h. cosolvents (e.g., 2-ethylhexyl ethoxylate, butyldiglycol,    propyldiglycol), more particularly in an amount of 0% to 5% by    weight, preferably of 0.1% to 2% by weight;-   i. corrosion control additives (e.g., nitrogen-containing    heterocycles, phosphoric esters, organic mono-, di-, and    tricarboxylic acids), more particularly in an amount of 0.01% to 5%    by weight, preferably of 0.1% to 2% by weight;-   j. complexing agents (e.g., aminoacetic acid derivatives, phosphonic    acid derivatives), more particularly in an amount of 0.01% to 5% by    weight, preferably of 0.1% to 1% by weight.

The amount of additional components (K) ought in total not to be morethan 30% by weight, more particularly not more than 20% by weight,preferably not more than 10% by weight, and with particular preferencenot more than 5% by weight (based in each case on the overallcomposition).

If metal ions or metal compounds are present, the compositions inquestion are preferably compositions which do not comprise chromiumcompounds. Furthermore, there ought preferably to be no metal fluoridesor complex metal fluorides present. The passivation of the invention,therefore, is preferably a chromium-free passivation, more preferably achromium- and fluoride-free passivation.

The aqueous compositions used in accordance with the invention may beobtained by mixing the components.

The metal surface that can be employed for treatment is in general anydesired metal surface, the surfaces in question being more particularlythose of base metals. These surfaces may be, for example, surfacescomprising or consisting substantially of iron, alloys of iron, steel,zinc (Zn), Zn alloys, aluminum (Al) or Al alloys, tin (Sn) and Snalloys, magnesium (Mg) or Mg alloys. The steels may be either low-alloyor high-alloy steels. Frequently the metal surface comprises aluminum oralloys of aluminum, or zinc or alloys of zinc, with a surface of zinc orof alloys of zinc being obtained generally by a galvanizing operation ona metallic material such as iron or steel.

The method of the invention is suitable more particularly forpassivating metallic surfaces of Zn, Zn alloys, Al or Al alloys. Thesesurfaces may be those of bodies or workpieces composed entirely of saidmetals and/or alloys. Alternatively they may be the surfaces of bodiescoated with Zn, Zn alloys, Al or Al alloys, it being possible for thebodies to be composed of other materials, as for example of othermetals, alloys, polymers or composite materials. The surface in questionmay more particularly be of galvanized iron or steel. The term“galvanized” also encompasses coating with a zinc alloy, more particularhot dip galvanizing with Zn—Al alloys, and electrogalvanizing withZn—Ni, Zn—Fe, Zn/Mn, and Zn/Co alloys.

The present invention relates preferably to a method for passivating ametallic surface which is a surface consisting substantially of one ormore metals selected from the group consisting of zinc (Zn), aluminum(Al), and magnesium (Mg).

Zn alloys or Al alloys are known to the skilled person. The desired endapplication influences the skilled person's choice of the type andamount of alloying constituents. Typical further constituents of zincalloys comprise more particularly Al, Mg, Pb, Si, Mg, Sn, Cu, and Cd.Also possible are Al/Zn alloys in which Al and Zn are present inapproximately the same amount. The coatings may be largely homogeneouscoatings or else coatings having concentration gradients. One possibleexample is galvanized steel which has additionally be vapor-coated withMg. As a result of this, a Zn/Mg alloy can be produced superficially.Typical further constituents of aluminum alloys are more particularlyMg, Mn, Si, Zn, Cr, Zr, Cu, and Ti.

In one preferred embodiment of the method the surface in question isthat of a strip metal, preferably comprising aluminum or an alloy ofaluminum, or comprising iron or steel, especially strips ofelectrogalvanized or hot dip galvanized steel.

The surfaces in question are often those of shaped bodies which areobtainable from said strip metals by processing procedures such ascutting, working and/or joining. Examples are automobile bodies or partsthereof, truck bodies, paneling for household appliances (such aswashing machines, dishwashers, wash driers, gas and electric cookers,microwave ovens, chest freezers or refrigerators, for example), claddingfor technical appliances or apparatus (such as machines, switchcabinets, computer housings or the like, for example), components in thearchitectural sector (such as wall parts, facade elements, ceilingelements, windows or door profiles or partitions), and furniture made ofmetallic materials (such as metal cabinets or metal shelving).

The metallic surfaces for treatment may also have thin oxidic,hydroxidic and/or carbonatic surface layers or layers of similarconstruction. Layers of this kind form usually spontaneously on metallicsurfaces in contact with the atmosphere, and are included in the term“metallic surface”.

In one preferred embodiment, the method relates to a continuous methodfor passivating galvanized steel strips on a coil-coating linesubsequent to galvanization (e.g., electrogalvanizing or hot dipgalvanizing).

The method of the invention can be used to passivate steel stripsgalvanized on one or both sides. Galvanized steel strips have athickness of 0.2 to 0.3 mm and widths of 0.5 to 2.5 m. Galvanized steelstrips are available commercially for a variety of applications. Theskilled person selects a suitable steel strip in accordance with thedesired end use.

Generally speaking, the method of the invention can be performed usingthe known coil-coating lines, based, for example, on squeeze-rolltechnology or on roll-coater technology. Suitable lines are described inWO 2009/047209, for example.

The method of the invention for passivating galvanized steel strips isperformed preferably by means of a continuous process using squeezerolls. For this purpose, the galvanized steel strip is moved by means ofdrive rollers. The passivating composition is applied to the steel stripby spraying, using a spraying station, for example, and forms a wetfilm. One or more squeeze rolls squeeze off excess passivatingcomposition. The result is a thin, wet film, which can be driedsubsequently in a drier.

The steel strips may be run through the line typically with a speed of80 to 200 m/min, preferably 50 to 150 m/min. The treatment time can bespecified by the skilled person in accordance with the desiredproperties of the passivation coat, and with other factors. In the caseof continuous processes it is advantageous for the maximum time betweenapplication of the aqueous composition to the metallic surface of thesteel strip and drying of the film to be 1 to 60 s.

In the method of the invention described above, the metallic surface maybe contacted with the aqueous composition, more particularly byspraying, dipping or roll application.

In one preferred embodiment, the method described above for passivatinga metallic surface comprises the following steps:

-   -   i) optionally cleaning the metallic surface to remove oils,        greases, dirt and/or oxide films;    -   ii) optionally washing the metallic surface with water;    -   iii) contacting a metallic surface with an aqueous composition        described above, the aqueous composition being applied in the        form of a wet film to the surface;    -   iv) drying the wet film obtained in step iii), preferably at        temperatures in the range from 20 to 250° C.;    -   v) optionally aftertreating the passivating surface.

In one preferred embodiment, the invention pertains to a method asdescribed above where the aqueous composition is applied to the metallicsurface in the form of a wet film, the aqueous composition being appliedat a coat weight in the range from 0.3 to 2 g/m², preferably 0.3 to 1g/m², more particularly 0.4 to 0.8 g/m² (based on the sum of the solidsof the aqueous composition) to the metallic surface.

The passivation coats obtainable with the described method of theinvention preferably have a coat thickness in the range from 1 to 3 μm,preferably 1 to 2 μm, and a coat weight in the range from 0.3 to 2 g/m²,preferably 0.3 to 1 g/m², with particular preference 0.4 to 0.8 g/m²(based on the sum of the solids in the aqueous composition).

In a further aspect, the present invention pertains to a composition forpassivating a metallic surface, comprising:

-   -   a) 10% to 40% by weight of at least one water-soluble        polymer (X) comprising acidic groups, the polymer (X) having at        least 0.6 mol of acid groups/100 g of polymer, and where        carboxyl groups and phosphonic acid groups are present        preferably as acidic groups;    -   b) 1% to 20% by weight of at least one volatile basic        compound (B) selected from ammonia, C1-4 alkylamines, and C1-4        alkanolamines;    -   c) 0.01% to 25% by weight of at least one cationic crosslinker        selected from polyvalent metal ions (M) and cationic polyamines        (P)    -   d) 20% to 89% by weight of at least one solvent (L), comprising        at least 80% by weight of water;    -   e) optionally 0% to 3% by weight of at least one surfactant (T),    -   f) optionally 0% to 30% by weight of at least one further        component (K);    -   the pH of the composition being in the range from 9 to 12,        preferably in the range from 9.5 to 11.

In one preferred embodiment, the invention is directed to a composition(more particularly an aqueous composition) for passivating a metallicsurface, comprising

-   -   a) 10% to 40% by weight of at least one water-soluble        polymer (X) comprising acidic groups, the polymer (X) containing        at least 0.6 mol of acid groups/100 g of polymer, and the        polymer (X) more particularly comprising carboxyl groups and        phosphonic acid groups;    -   b) 1% to 20% by weight of ammonia;    -   c) 0.5% to 5% by weight of at least one polyamine (P) selected        from polyethyleneimines, polyethyleneimine derivatives,        polyvinylamines, and polyvinylimidazoles;    -   d) 20% to 89% by weight of at least one solvent (L), comprising        at least 80% by weight of water;    -   e) optionally 0.01% to 25% by weight of at least one polyvalent        metal ion (M) selected from Mg²⁺, Ca²⁺, Zn²⁺, Mn²⁺, Fe²⁺, Cr³⁺,        Al³⁺, Ti⁴⁺, Zr⁴⁺, and Ce³⁺;    -   f) optionally 0% to 3% by weight of at least one surfactant (T);    -   the composition having a pH in the range from 9 to 12,        preferably in the range from 9.5 to 11.

For the stated components (polymer (X), volatile base (B), polyamine(P), metal ion (M), surfactant (T), solvent (L), and further components(K)), the preferred embodiments stated earlier on above in connectionwith the method of the invention are applicable.

The present invention is further directed to the use of theabove-described composition for passivating a metallic surface, moreparticularly a metallic surface consisting substantially of one or moremetals selected from the group consisting of zinc (Zn), aluminum (Al)and magnesium (Mg), more particularly of galvanized steel. The inventionrelates more particularly to the use of the above-described compositionfor passivating a galvanized steel strip in a continuous process.

The present invention further provides a coating on a metallic surface,obtainable by contacting the metallic surface with an aqueouscomposition comprising the following components:

-   -   a) at least one water-soluble polymer (X) comprising acidic        groups, the polymer (X) having at least 0.6 mol of acid        groups/100 g of polymer;    -   b) at least one volatile basic compound (B),    -   c) at least one cationic crosslinker selected from polyvalent        metal ions (M) and cationic polymers (P);    -   d) at least one solvent (L), comprising at least 80% by weight        of water;    -   e) optionally one surfactant (T);    -   f) optionally further components (K);

the pH of the aqueous composition being in the range from 9 to 12,preferably in the range from 9.5 to 11.

For the stated components (polymer (X), volatile base (B), cationicpolymer (P), metal ion (M), surfactant (T), solvent (L), and furthercomponents (K)), the preferred embodiments stated earlier on above inconnection with the method of the invention are applicable.

The invention is directed more particularly to an above-describedcoating wherein the coating additionally has one or morecoating-material coats.

The present invention is elucidated in more detail with reference to theexperimental examples below.

EXAMPLE 1

Use was made in each case of aqueous solutions of anacid-group-containing polymer (polymer X) of about 50% by weight ofacrylic acid (monomer M1), about 30% by weight of vinylphosphonic acid(monomer M2), and about 20% by weight of hydroxyethyl acrylate (monomerM3), prepared as described in WO 2008/612248.

Initial formulations were prepared from about 20% by weight of theacid-group-containing polymer (polymer (X)), optionally 85% phosphoricacid (H₃PO₄) and zinc oxide (ZnO), by dissolving or dispersing thecomponents in water, where necessary, and mixing the components. Theprecise quantities (in % by weight) of the components in the initialformulation are summarized in table 1.

The various initial formulations were adjusted to a pH of approximately10.5 by addition of ammonia (volatile basic compound B). Moreover, incertain formulations, additionally, a polyethyleneimine having anaverage molecular weight of 2000 g/mol, as cationic crosslinker, wasadded. In formulation V4, additionally, a surfactant and a defoamer(modified silane) were used. As reference C1, a hot dip galvanized metalsheet without passivation was used. Formulations V1 to V4 below wereproduced in accordance with the details in table 1.

TABLE 1 Coating formulations C1 V1 V2 V3 V4 Polymer (X) — 15 15 15 15Polyethyleneimine (component (P)) — — 5 5 NH₃ (25% strength) (component(B)) — 10 15 15 15 H₃PO₄ — — 6 6 6 ZnO (component (M)) —  2 2 2 2Surfactant (T) — — — — 0.5 Defoamer — — — — 0.05

EXAMPLE 2

Using the compositions described in example 1, coated test panels of hotdip galvanized steel (Gardobond OE HDG 3, 105×190 mm) were produced.

As a pretreatment, the test panels were immersed for around 30 secondsin a milk alkaline cleaner solution (Surtech 133 from Surtech),immediately rinsed off with fully demineralized water, and then driedwith nitrogen. The cleaned panels were immersed at room temperature for1 second (s) each in the formulations listed in table 1, squeezed offwith a roller system, and dried in a drying cabinet at 160° C. for 12seconds.

The peak metal temperature (PMT) in the course of drying here did notexceed 50° C.

The coated test panels thus obtained were tested for their corrosionresistance. Tests carried out were the salt spray test, the condensationcycling test (CCT), and the stack test, as described in example 3.

The test results are summarized in table 2.

TABLE 2 Test results for corrosion resistance X1 Reference V1 V2 V3 V4pH of solution — — 10.5 10.4 10.5 10 Coat weight [g/m²] 0 0.8 0.8 0.80.8 Salt spray test 24 h 0 6 9 10 10 DIN ISO 9227 48 h — 3 7 8 9 72 h —0 3 5 6 96 h — — 0 0 3 CCT 21 cycles 4 4 2 1 1 DIN ISO 6270-2 Stack test 1 day 4 0 0 0 0  7 days — 3 2 1 1 14 days — 4 4 4 3 Appearance of clearclear clear clear clear solution

It is apparent that coatings with the inventive compositions exhibitedsignificantly better corrosion resistance in all test methods incomparison to test panels coated with a comparative solution.

EXAMPLE 3

The passivation coat was assessed by performance of the test proceduresdescribed below and summarized in table 2.

a) Salt Spray Test (DIN EN ISO 9227)

The quality of the corrosion control on the test panels coated inaccordance with example 2 was evaluated in the neutral salt spray testas per DIN EN ISO 9227, by awarding evaluation scores of 0 to 10 inaccordance with defined standards.

The evaluation score or degree of evaluation is a measure of theformation of white rust on the panel. The higher the evaluation score,the lower the proportion of the corroded surface area [%] and the betterthe corrosion control. The evaluation scores were awarded in accordancewith table 3. An average was formed from 5 panels.

TABLE 3 Salt spray test evaluation scheme Corroded area [%] Evaluationscore No defects 10   0 < A < 0.1 9  0.1 < A < 0.25 8 0.25 < A < 0.5  70.5 < A < 1.0 6 1.0 < A < 2.5 5 2.5 < A < 5.0 4 5.0 < A < 10  3 10 < A <25 2 25 < A < 50 1 50 < A 0

b) Condensation Cycling Test CCT (DIN EN ISO 6270-2)

Additionally, the panels were tested in the condensation cycling test“CCT” in accordance with DIN EN ISO 6270-2. This test consists of one ormore cycles each with two test sections.

In the first section, the test specimens are exposed for 8 hours to atemperature of 40° C. and a relative humidity of 100%; in the secondsection, they are exposed to a temperature of 18-28° C. and a humidityof below 100% (ambient conditions). The duration of a cycle is 24 hours.The samples were assessed visually in accordance with the followingcriteria, as an average of 3 panels:

-   -   0 no chalking    -   1 slight chalking    -   2 moderate chalking    -   3 severe chalking    -   4 very severe chalking

The resistance of the test panels coated as described above was comparedagainst one another in a stack test.

For this purpose, three coated panels were divided in the center, wettedwith 5 ml of distilled water each, and placed with the test sidesagainst one another. Testing took place in a climatic cycling chamber,in which the stacks were weighted with a 5 kg weight and exposed to adefined number of cycles (DIN EN ISO 6270-2, AHT).

The corrosion was evaluated as the average over 3 panels, in accordancewith the following evaluation scheme:

-   -   0=no change relative to fresh panel, visually satisfactory    -   1=incipient whitening/chalking (0-20% of the area white)    -   2=significant whitening/chalking and incipient white rust        (20-50% of the area white)    -   3=severe whitening/chalking (50-80% of the area white)    -   4=completely corroded (80-100% of the area white)

d) Determination of Coat Weight

The coat weight was determined gravimetrically by measuring thedifference between the weights before and after coating, in each casewith the test panels in their demagnetized and dry condition. Theweights are then converted to account for the area of the respectivepanels, and reported in [g/m²].

1. A method for passivating a metallic surface, wherein a metallicsurface is contacted with an aqueous composition comprising thefollowing components: a) at least one water-soluble polymer (X)comprising acidic groups, the polymer (X) having at least 0.6 mol ofacid groups/100 g of polymer; b) at least one volatile basic compound(B); c) at least one cationic crosslinker selected from polyvalent metalions (M) and cationic polymers (P); d) at least one solvent (L),comprising at least 80% by weight of water; e) optionally one or moresurfactants (T); f) optionally further components (K); the pH of theaqueous composition being in the range from 9 to
 12. 2. The methodaccording to claim 1, wherein a composition is used comprising: a) 10%to 40% by weight of at least one water-soluble polymer (X) comprisingacidic groups, the polymer (X) having at least 0.6 mol of acidgroups/100 g of polymer; b) 1% to 20% by weight of at least one volatilebasic compound (B); c) 0.01% to 25% by weight of at least one cationiccrosslinkers selected from polyvalent metal ions (M) and cationicpolymers (P); d) 20% to 89% by weight of at least one solvent (L),comprising at least 80% by weight of water, e) optionally 0% to 3% byweight of at least one surfactant (T), f) optionally 0% to 30% by weightof at least one further component (K). the pH of the aqueous compositionbeing in the range from 9 to
 12. 3. The method according to claim 1,wherein the aqueous composition comprises as cationic crosslinker atleast one polyvalent metal ion (M) selected from Mg²⁺, Ca²⁺, Zn²⁺, Mn²⁺,Fe²⁺, Cr³⁺, Al³⁺, Zr⁴⁺, Ce³⁺, and Ce⁴⁺.
 4. The method according to claim1, wherein the aqueous composition comprises as cationic crosslinker atleast one cationic polymer (P) selected from polyethyleneimines,polyethyleneimine derivatives, polyvinylamines, and polyvinylimidazoles.5. The method according to claim 1, wherein the composition comprises0.1% to 2% by weight, preferably 0.1% to 0.8% by weight, of at least onesurfactant (T).
 6. The method according to claim 1, wherein thewater-soluble polymer (X) is a copolymer (X1) synthesized from thefollowing monomers: M1: 30% to 90% by weight of methacrylic acid and/oracrylic acid; M2: 10% to 70% by weight of at least one furthermonoethylenically unsaturated monomer, different from (M1), which hasone or more acidic groups; optionally M3: 0% to 40% by weight of atleast one OH-containing methacrylic ester and/or acrylic ester;optionally M4: 0% to 30% by weight of at least one further ethylenicallyunsaturated monomer, different from (M1), (M2) or (M3).
 7. The methodaccording to claim 1, wherein the water-soluble polymer (X) is acopolymer which is synthesized from the following monomers: M1: 20% to60% by weight of acrylic acid; M2: 20% to 60% by weight ofvinylphosphonic acid; M3: 1% to 40% by weight of hydroxyethyl acrylate.8. The method according to claim 1, wherein the aqueous composition isapplied to the metallic surface in the form of a wet film and theaqueous composition is applied at a coatweight in the range from 0.3 to2 g/m² to the metallic surface.
 9. A composition for passivating ametallic surface, comprising a) 10% to 40% by weight of at least onewater-soluble polymer (X) comprising acidic groups, the polymer (X)having at least 0.6 mol of acid groups/100 g of polymer; b) 1% to 20% byweight of at least one volatile basic compound (B) selected fromammonia, C₁₋₄ alkylamines, and C₁₋₄ alkanolamines; c) 0.01% to 25% byweight of at least one cationic crosslinker selected from polyvalentmetal ions (M) and cationic polymers (P) d) 20% to 89% by weight of atleast one solvent (L), comprising at least 80% by weight of water; e)optionally 0% to 3% by weight of at least one surfactant (T); f)optionally 0% to 30% by weight of at least one further component (K);the pH of the aqueous composition being in the range from 9 to
 12. 10.The composition according to claim 9, comprising a) 10% to 40% by weightof at least one water-soluble polymer (X) comprising acidic groups, thepolymer (X) containing at least 0.6 mol of acid groups/100 g of polymer;b) 1% to 20% by weight of ammonia; c1) 0.5% to 5% by weight of at leastone cationic polymer (P) selected from polyethyleneimines,polyethyleneimine derivatives, polyvinylamines, and polyvinylimidazoles;c2) optionally 0.01% to 25% by weight of at least one polyvalent metalion (M) selected from Mg²⁺, Ca²⁺, Zn²⁺, Mn²⁺, Fe²⁺, Cr³⁺, Al³⁺, Zr⁴⁺,and Ce³⁺; d) 20% to 89% by weight of at least one solvent (L),comprising at least 80% by weight of water; e) optionally 0% to 3% byweight of at least one surfactant (T); f) optionally 0% to 30% by weightof at least one further component (K); the pH of the aqueous compositionbeing in the range from 9 to
 12. 11. A coating on a metallic surface,obtainable by contacting a metallic surface with an aqueous compositioncomprising the following components: a) at least one water-solublepolymer (X) comprising acidic groups, the polymer (X) having at least0.6 mol of acid groups/100 g of polymer; b) at least one volatile basiccompound (B); c) at least one cationic crosslinker selected frompolyvalent metal ions (M) and cationic polymers (P); d) at least onesolvent (L), comprising at least 80% by weight of water; e) optionallyone surfactant (T); f) optionally further components (K), the pH of theaqueous composition being in the range from 9 to 12.